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6 Minerals A number of inorganic elements are essential for normal growth and reproduction of animals. Those required in gram quantities are referred to as macrominerals and this group includes calcium, phosphorus, sodium, chlorine, po- tassium, magnesium, and sulfur. The macrominerals are important structural components of bone and other tissues and serve as important constituents of body fluids. They play vital roles in the maintenance of acid-base balance, osmotic pressure, membrane electric potential and nervous transmission. Those elements required in milligram or microgram amounts are referred to as the trace minerals. This group includes cobalt, copper, iodine, iron, manga- nese, molybdenum, selenium, zinc, and perhaps chromium and fluorine. Other elements have been suggested to be essential based on studies in other species but these are generally not considered to ever be of practical importance in dairy cattle. The trace minerals are present in body tissues in very low concentrations and often serve as com- ponents of metalloenzymes and enzyme cofactors, or as components of hormones of the endocrine system. A facto- rial approach was used to describe the requirements for both the macro- and trace minerals whenever such an approach could be supported by research data. Maintenance requirements as described in this model will include the endogenous fecal losses and insensible urinary losses. Though technically not correct, it will also include losses incurred through sweat. The lactation requirement will be defined as the concentration of the mineral in milk multiplied by the 4 percent FCM milk yield. The pregnancy requirement is defined as the amount of mineral retained within the reproductive tract (fetus, uterine contents, and uterus) at each day of gestation. For most minerals, the requirement of the animal pregnant for <190 days is small and not considered in the model. The growth requirement is expressed as the amount of mineral retained/kg body weight gained and entered into the model as expected average daily gain (ADG). The sum of the maintenance, lactation, pregnancy, and growth requirements is the true requirement of the tissues for the mineral, and is referred to as the "requirement for absorbed mineral." The diet must supply this amount to the tissues. Not all the mineral in a diet is available for absorption. Where data permitted, the availability of min- erals from forages, concentrates, and inorganic sources was assigned an absorption coefficient. The model evaluates the absorbable mineral content of a diet by determining the available mineral provided by each constituent of the diet and comparing the sum of the amount of mineral available from the diet with the requirement of the animal for absorbed mineral. For all minerals considered essential, detrimental effects on animal performance can be demonstrated from feeding excessive amounts. Generally, the dietary amount required for optimal performance is well below amounts found to be detrimental to performance. However, toxicity from several of the essential minerals, including fluorine, sele- nium, molybdenum, and copper are unfortunately prob- lems that can occur under practical feeding conditions. The National Research Council (1980) described signs of toxicosis and the dietary concentrations of minerals that are considered excessive. Certain elements such as lead, cadmium, and mercury are discussed because they should always be considered toxic and are of practical concern because toxicosis from these elements unfortunately occa- sionally occurs. Concentrations of mineral elements in both concentrate and forage foodstuffs vary greatly (Adams, 1975; Coppock and Fettman, 1977; Kertz, 19981. Reliable or typical analy- ses of concentrations of some mineral elements (e.g., chlo- ride and various micromineral elements) in many foodstuffs are unavailable (Henry, 1995c). A1SO, concentrations among samples of the same feed type may be quite variable depending upon such factors as fertilization and manure application rates, soil type, and plant species (Butler and Jones, 19731. Concentrations in byproducts or coproducts also are variable and influenced by the method of process- ing to produce the feedstuff. Therefore, laboratory analyses of feeds for macro- and micromineral element content is 105

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106 Nutrient Requirements of Dairy CattIe critically important for precise and accurate diet formula- tion to meet requirements at least cost. Laboratory analyses using wet chemistry methods is critical for accurate deter- mination. Near infrared reflectance spectroscopy (NIRS) is not reliable (Shenk and Westerhaus, 19941. Estimates of mean concentrations and of variation (stan- dard deviations) in mineral element content of many com- monly used foodstuffs are given in Table 15-1 of this publi- cation. Compositions of inorganic mineral sources com- monly used in diet supplementation are presented in Table 15-3. MAC RO MINE RALS Calcium FUNCTIONS Extracellular calcium is essential for formation of skeletal tissues, transmission of nervous tissue impulses, excitation of skeletal and cardiac muscle contraction, blood clotting, and as a component of milk. Intracellular calcium, while 1/lO,OOO of the concentration of extracellular calcium, is involved in the activity of a wide array of enzymes and serves as an important second messenger conveying infor- mation from the surface ofthe cell to the interior ofthe cell. About 98 percent of the calcium in the body is located within the skeleton where calcium, along with phosphate anion, serves to provide structural strength and hardness to bone. The other 2 percent of the calcium in the body is found primarily in the extracellular fluids of the body. Normally the concentration of calcium in blood plasma is 2.2 to 2.5 mM (9 to 10 mg/dl, or 4.4 to 5 mEq/L) in the adult cow, with slightly higher values in calves. Between 40 and 45 percent of total calcium in plasma is bound to plasma proteins, primarily albumin, and another 5 percent is bound to organic components of the blood such as citrate or inorganic elements. From 45 to 50 percent of total calcium in plasma exists in the ionized, soluble form; the amount being closer to 50 percent at low blood pH and closer to 45 percent when blood pH is elevated. The ionized calcium concentration of the plasma must be maintained at a relatively constant value of 1 to 1.25 mM to ensure normal nerve membrane and muscle end plate electric potential and conductivity, which has forced vertebrates to evolve an elaborate system to maintain calcium homeo- stasis. This system attempts to maintain a constant concen- tration of extracellular calcium concentration by increasing calcium entry into the extracellular fluids whenever there is a loss of calcium from the extracellular compartment. When the loss of calcium exceeds entry, hypocalcemia can occur and this results in loss of nerve and muscle function, which can in some instances lead to recumbency and the clinical condition referred to as milk fever. During vitamin D intoxication, calcium enters the extracellular compart- ment faster than it leaves resulting in hypercalcemia, which can lead to soft tissue deposition of calcium. CALCIUM HOMEOSTASIS Calcium leaves the extracellular fluids during bone for- mation, in digestive secretions, sweat, and urine. An espe- cially large loss of calcium to milk occurs during lactation in the cow. Calcium lost via these routes can be replaced from dietary calcium, from resorption of calcium stored in bone, or by resorbing a larger portion of the calcium filtered across the renal glomerulus, i.e., reducing urinary calcium loss. Whenever the loss of calcium from the extracellular fluids exceeds the amount of calcium entering the extracel- lular fluids there is a decrease in the concentration of calcium in plasma. The parathyroid glands monitor the concentration of calcium in carotid arterial blood and secrete parathyroid hormone when they sense a decrease in blood calcium. Parathyroid hormone immediately increases renal reabsorption mechanisms for calcium to reduce the loss of urinary calcium, and will stimulate pro- cesses to enhance intestinal absorption of calcium and resorption of calcium from bone. Ultimately dietary calcium must enter the extracellular fluids to permit optimal performance of the animal. Cal- cium absorption can occur by passive transport between epithelial cells across any portion of the digestive tract whenever ionized calcium in the digestive fluids directly over the mucosa exceeds 6 mM (Bronner, 19871. These concentrations are reached when calves are fed all milk diets and when cows are given oral calcium drenches for prevention of hypocalcemia (Goff and Horst, 19931. In nonruminant species, studies suggest that as much as 50 percent of dietary calcium absorption can be passive (Nel- lans, 19881. It is unknown how much passive absorption of calcium occurs from the diets typically fed to dairy cattle but the diluting effect of the rumen would likely reduce the degree to which passive calcium absorption would occur. Active transport of calcium appears to be the major route for calcium absorption in mature ruminants and this pro- cess is controlled by 1,25-dihydroxyvitamin D, the hormone derived from vitamin D. By carefully regulating the amount of 1,25-dihydroxyvitamin D produced, the amount of dietary calcium absorbed can be adjusted to maintain a constant concentration of extracellular calcium (DeLuca, 1979; Bronner, 1987; Wasserman, 19811. When dietary calcium is insufficient to meet the require- ments of the animal, calcium will be withdrawn from bone to maintain a normal concentration of extracellular calcium. If dietary calcium is severely deficient for a prolonged period the animal will develop severe osteoporosis to the point of developing fractures still, because the desire to maintain a normal concentration of extracellular calcium

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Minerals 107 is so strong, plasma calcium will only be slightly lower than normal. A sudden large . 1 r 1 . 1 ~~ ~ ~ increase in loss of calcium from the extracellular pool can result in acute hypocalcemia before the calcium homeostatic mechanisms can act. This is discussed further in the section on milk fever (Chapter 91. REQUIREMENT FOR ABSORBED CALCIUM The amount of calcium that must enter the extracellular compartment for maintenance, growth, pregnancy, and lac- tation is fairly well known and essentially the same equa- tions were used to predict these amounts as were used in the 1989 National Research Council publication Nutrients Requirements of Dairy Cattle. Maintenance For maintenance of nonlactating cattle, the absorbed calcium required is 0.0154 g/kg body weight (Visek et al., 1953; Hansard et al., 19571. For lactating animals the maintenance requirement is increased to 0.031 g/kg live BW (Martz et al., 19901. The increase in lactating cows reflects the impact increased dry matter intake (DMI) has on intestinal secretion of calcium during digestion. Growth Growth of cattle requires more calcium when animals are young and actively accruing bone and less as they reach mature skeletal size. The Agricultural and Food Research Council (1991) developed an allometric equation to describe the calcium requirement of growing calves which will be adopted in this model. The requirement for absorbed calcium/kg average daily gain is: Ca (g/day) = (9.83 x (MW022) x (BW-0221) x WG where MW = expected mature live body weight (kg), BW = current body weight, and WG = weight gain. Pregnancy The developing fetus requires a negligible amount of calcium until the last trimester of pregnancy (after day 190 of pregnancy), when the fetal skeleton begins to become calcified. Fetal skeletal calcification is especially great in the last weeks before parturition. The absorbed calcium required to meet the demands of the uterus and conceptus is best described by the exponential equation of House and Bell (House and Bell, 1993) for any given day of gestation beyond day 190 as: Ca (g/day) = 0.02456 e`00558~-00000 0.02456 e(005581-0.00007~-~_~) where t = day of gestation. Lactation The amount of calcium/kg milk produced var- ies slightly with the amount of protein in the milk which in turn varies with breed. The absorbed calcium required/ kg milk produced is 1.22 g for Holstein cows, 1.45 g for Jersey cows, and 1.37 g for other breeds. Cows require about 2.1 g absorbed C a/kg of colostrum produced. CALCIUM ABSORPTION COEFFICIENT The amount of calcium that must be fed to meet the requirement for absorbed calcium is dependent on the availability of calcium from the foodstuffs and inorganic calcium sources in the diet, and the efficiency of intestinal calcium absorption in the animal being fed. The amount of calcium absorbed from the diet will generally equal the requirement of the body for calcium if the diet contains enough available calcium. The proportion of dietary cal- cium absorbed will decrease as dietary calcium increases above requirement of the tissues for absorbed calcium. To truly determine the efficiency of absorption of calcium from a feedstuff, the animals being tested should be fed less total dietary calcium than the amount of absorbed calcium required to meet their needs. This will ensure that intestinal calcium absorption mechanisms are fully activated so that the animal will absorb all the calcium from the foodstuff that it possibly can. Few studies fulfill this requirement; thus, it is likely that the published data have underestimated the availability of calcium in many cases. Previous National Research Council (1978, 1989) publications have determined a single efficiency of absorp- tion of dietary calcium regardless of the source of calcium. This absorption coefficient was 0.38 in the 1989 Nutrient Requirements of Dairy Cattle and 0.45 in the 1978 Nutrient Requirements of Dairy Cattle based on the average propor- tion of calcium absorbed during a variety of trials. The coefficient was reduced in the 1989 Nutrient Requirements of Dairy Cattle partly in response to reports that cows in early lactation were less able to utilize dietary calcium (Van's Klooster, 1976; Ramberg, 1974) making use of a lower coefficient for calcium absorption more prudent. The decision to utilize 0.38 as the calcium absorption coefficient was based largely on a summary of 11 experiments with lactating dairy cows in which the average percentage of dietary calcium absorbed was 38 (Hibbs and Conrad, 19831. In the majority of these 11 experiments, the cows were fed diets supplying calcium well in excess of their needs placing the cows in positive calcium balance by as much as 20 to 40 g/day. In 3 of the experiments, the cows were in negative calcium balance and the percentage of dietary calcium absorbed was still below 40 percent. In those experiments, alfalfa and/or brome hay were supplying the dietary calcium. The French Institut National de la Recher- che Agronomique (1989) used 30 to 35 percent as an esti- mate of efficiency of absorption for dietary calcium using similar logic. The 1996 Nutrient Requirements of Beef Cat- tle utilized 50 percent as the calcium absorption co- eff~cient. The 1980 United Kingdom Agricultural Research Council (Agricultural Research Council, 1980) chose 68

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108 Nutrient Requirements of Dairy CattIe percent as the coefficient of absorption for calcium; a coef- ficient considerably higher than the estimate of other groups that had examined dietary calcium requirements of cattle. This number is based on a model that predicts calcium is absorbed by dairy cattle according to need. Using data from a variety of balance studies in which a substantial number of lactating cows was included, the observed eff~- ciency of absorption of calcium (diets utilizing foodstuff and mineral sources of calcium) reached a plateau of about 68 percent. Concern over the validity of this coefficient prompted the Agricultural Research Council (1980) to form a second committee to review the 1980 recommenda- tion for calcium. This technical committee agreed with the use of 68 percent as an estimate of the absorption coeff~- cient to be used in calculating dietary calcium requirements of cattle (AFRC, 19911. A single coefficient is inappropriate and in this model the coefficient for calcium absorption will be based on the sources of calcium used in the diet. Unfortunately, our knowledge of the efficiency of absorption of calcium from individual foodstuffs is limited. Martz et al. (1990) fed lactating dairy cows two diets with no added mineral sources of calcium in which alfalfa supplied nearly all of the dietary calcium. One diet was 33 percent alfalfa, 39 percent hominy grits and 21.5 percent corn cobs; the sec- ond diet was 24 percent alfalfa, 41.5 percent corn silage and 29 percent hominy grits. The diets contained more calcium than suggested by the 1978 National Research Council Nutrient Requirements of Dairy Cattle publication and less than suggested by the 1989 National Research Council Nutrient Requirements of Dairy Cattle. True absorption of calcium from alfalfa, corrected for endoge- nous fecal calcium loss, was 25 percent; whereas, from the alfalfa-corn silage ration 42 percent of calcium was truly absorbed. Ward et al. (1972) estimated the efficiency of absorption of calcium from alfalfa ranged from 31 to 41 percent. About 20 to 30 percent of calcium within plants is bound to oxalate which is relatively unavailable to the ruminant (Ward et al., 19791. Studies of Hibbs and Conrad (1983) where cows were in negative calcium balance and were fed only alfalfa or alfalfa/brome diets fit the criteria of determining calcium absorption in animals that are being fed less calcium than they require and in these studies the efficiency of absorption of calcium from alfalfa ranged from 8 to 37 percent. Because alfalfa is a major contributor of calcium in dairy rations, absorption of calcium from alfalfa is used as an estimate of efficiency of absorption of calcium from forages in general. An efficiency of absorption of 30 percent is used in the model for calcium from forages. Availability of calcium from grains and concentrates has not been determined in ruminants. In nonruminant ani- mals, the availability of calcium from concentrates gener- ally is less than the availability of calcium from an inorganic source such as calcium carbonate (Soares, 19951. The pres- ence of phytate is felt to be a factor impairing absorption in nonruminants. This is not a factor in ruminants. Because oxalate is not as likely in concentrate foodstuffs the propor- tion of calcium available should be greater than 30 percent. It is possible that it may be comparable to that of the mineral sources of calcium. However the current model uses 60 percent as a conservative estimate of the proportion of calcium available from concentrate foodstuffs based in part on an assumption that the availability is not as high as from calcium carbonate. Efficiency of absorption of calcium from foodstuffs that are not forages (e.g., concen- trates) was set at 60 percent. Most non-forage foodstuffs will contain only small amounts of calcium. However, a notable exception is the calcium soap of palm oil fatty acids, which can be 7 to 9 percent calcium. The fat of this product is approximately 80 percent digestible, and digestion can only occur following dissociation of the calcium from the palmitate in the small intestine. This also implies that 80 percent of the calcium in this feed ingredient is available for absorption. This is in contrast to the work of Oltjen (1975) which suggested that formation of calcium soaps within the rumen impaired calcium absorption necessitating an increase in diet calcium when fat was added to a ration. No effect of added fat on apparent absorption of calcium was observed in the experiments of Rahnema et al. (19941. The model does not include a factor to increase dietary calcium when fat is added to the diet. There may be a need to increase diet magnesium when fat is added to the diet as magnesium must be soluble in the rumen to be absorbed. Since hypo- magnesemia can affect calcium metabolism (see Chapter 9) there is an effect of diet fat on calcium metabolism but it is not overcome by adding calcium to the ration. Calcium within mineral supplements is generally more available than calcium in forages and common foodstuffs (Hansard et al., 19571. Theoretically the factor limiting mineral calcium absorption is the solubility of the calcium from the mineral source. Calcium chloride represents a source of highly soluble calcium. When 45CaCl was used as a source of radioactive tracer for calcium absorption studies it was absorbed with >95 percent efficiency in young calves (Hansard et al., 19541. Calcium chloride is assigned an efficiency of absorption coefficient of 95 per- cent. Estimates of the efficiency of absorption of calcium from calcium carbonate range from 40 percent or 51 per- cent (Hansard et al., 1957) or up to 85 percent (Goetsch and Owens, 19851. Unfortunately, these studies were con- ducted using steers, with very low requirements for absorbed calcium. The studies of Hansard et al. (1957) demonstrate that calcium chloride is between 1.2 and 1.32 times more absorbable than calcium carbonate. Therefore, the efficiency of absorption of calcium from calcium car- bonate is designated to be 75 percent. The absorption of calcium from various mineral sources is often compared

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Minerals 109 to the efficiency of absorption of calcium from calcium carbonate. Table 15-4 lists a number of common mineral sources of calcium (including bone meal) and an estimate of the efficiency of absorption of calcium in each source, using data summarized by Soares (1995a) and based on the efficiency of absorption relative to calcium carbonate. The calcium from limestone generally is slightly less avail- able than from pure calcium carbonate and has been assigned an efficiency of absorption coefficient of 70 percent. EFFECTS OF PHYSIOLOGIC STATE The amount of available calcium that will actually be absorbed varies with the physiologic state of the animal. Hansard et al. (1954) and Horst et al. (1978) reported that the efficiency of absorption of calcium decreases as animals age. Young animals absorb calcium very efficiently and very old animals absorb calcium poorly. As animals age, there is a decline in vitamin D receptors in the intestinal tract (Horst et al., 1990), which is thought to reduce the ability to respond to 1,25-dihydroxyvitamin D. From the data of Hansard et al. (1954), the difference in efficiency of calcium absorption in beef steers from 1 to 6 years of age is nearly negligible. Age was not included as a factor to adjust dietary calcium requirement in cattle >200 kg body weight. The absorption coefficient for calcium from diets normally fed to calves is high and will be considered to be 90 percent for all calves <100 kg body weight (see calf section, Chapter 101. In early lactation nearly all cows are in negative calcium balance (Ellenberger et al., 1931; Ender et al.1971; Ramb- erg, 19741. As feed intake increases and calcium intake increases most cows go into positive calcium balance about 6 to 8 weeks into lactation (Hibbs and Conrad, 1983; Ellenberger et al., 19311. Cows in the first 10 days of lactation are at greatest risk of being in negative calcium balance (Ramberg, 1974) and some are subclinically hypo- calcemic throughout this period (Goff et al., 19961. Ram- berg (1974) reported that the rate of entry of calcium into the extracellular fluid pool from the intestine increased about 1.55-fold from the day before parturition until 10 days in milk. Thereafter, the rate of entry of calcium into the extracellular pool from the intestine was not increased any further. Van't Klooster (1976) demonstrated that cal- cium absorption increased from 22 percent in late gestation to 36 percent by day 8 of lactation after which it remained relatively constant. This represented a 1.6-fold increase in efficiency of calcium absorption over this 8-day period. Regression analysis of data of Ward et al. (1972) predicted that cows need to be fed 5 g C a/kg milk in early lactation to avoid negative calcium balance. However, there was no evidence to demonstrate that negative calcium balance in early lactation was detrimental to the cow provided the concentration of calcium in plasma remained normal, i.e., lactational osteoporosis ensures adequate entry of calcium from bone into the extracellular calcium pool. During lacta- tional osteoporosis, data of Ellenberger et al. (1931) suggest 800 to 1300 g of calcium are removed from bone to support milk production during early lactation and this calcium is restored to bone during the last 20 to 30 weeks of lactation and the dry period. This could increase the requirement for absorbed calcium in later lactation by as much as 8 g/d to rebuild bone lost during early lactation. No calcium requirement for rebuilding bone is included in the model. The effects of calcium-to-phosphorus ratio on absorption of calcium and phosphorus was once felt to be important but recent data suggest that the calcium: phosphorus ratio is not critical, unless the ratio is >7:1 or <1:1 (Miller, 1983a; Agricultural Research Council, 19801. CALCIUM DEFICIENCY A deficiency of dietary calcium in young animals leads to a failure to mineralize new bone and contributes to retarded growth. Rickets is more commonly caused by a deficiency of vitamin D or phosphorus but a deficiency of calcium can contribute to rickets as well. In older animals a deficiency of dietary calcium forces the animal to withdraw calcium from bone for homeostasis of the extracellular fluids. This causes osteoporosis and osteomalacia in the bones, which makes the bone prone to spontaneous frac- tures. The concentration of calcium in milk is not altered even during a severe dietary deficiency of calcium (Becker et al., 19331. EXCESS DIETARY CALCIUM Feeding excessive dietary calcium is generally not associ- ated with any specific toxicity. Dietary concentrations of calcium >1 percent have been associated with reduced DMI and lower performance (Miller, 1983a) but diets as high as 1.8 percent calcium have been fed with no apparent problems for nonlactating dairy cows (Beede et al., 19911. Feeding excessive calcium could interfere with trace min- eral absorption (especially zinc) and replaces energy or protein the animal might better utilize for increased pro- duction. Feeding calcium in excess of requirements has been suggested to improve performance, especially when cows are fed corn silage diets. Because calcium is a strong cation, addition of calcium carbonate to diets above that required to meet absorbed calcium needs may be providing a rumen alkalinizing effect to enhance performance. Phosphorus Of all dietary essential mineral elements for dairy ani- mals, phosphorus represents the greatest potential risk if

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110 Nutrient Requirements of Dairy Cattle excess is released into the environment contaminating sur- face waters and causing eutrophication. Accurate and pre- cise management of phosphorus nutrition is crucial to opti- mize performance and health of dairy animals, and to minimize phosphorus excretion. PHYSIOLOGIC ROLES Phosphorus has more known biologic functions than any other mineral element. About 80 percent of phosphorus in the body is found in bones and teeth. It is present in bone, along with calcium, principally as apatite salts, and as calcium phosphate. It is located in every cell of the body and almost all energy transactions involve formation or breaking of high-energybonds that link oxides of phosphate to carbon or to carbon-nitrogen compounds (such as adeno- sine triphosphate, ATP). Phosphorus also is intimately involved in acid-base buffer systems of blood and other bodily fluids, in cell differentiation, and is a component of cell walls and cell contents as phospholipids, phosphopro- teins, and nucleic acids. Phosphorus concentrations in blood plasma normally are 1.3 to 2.6 mmol/L (4 to 8 mg/dl; 6 to 8 mg/dl for growing cattle and 4 to 6 mg/dl for adult animals). About 1 to 2 g circulate as inorganic phosphate in blood plasma of a 600-kg animal. Because of greater concentrations in erythrocytes, whole blood contains 6 to 8 times as much phosphorous as plasma. About 5 to 8 g are present in the extracellular pool of a 600-kg cow. The intracellular concentration of phosphorus is about 25 mmol/L (78 mg/dl), and total intra- cellular phosphorus is about 155 ~ in a 600-k~ cow (Goff, 1998a). _, , , O O Phosphorus also is required by ruminal microorganisms for digestion of cellulose (Burroughs et al., 1951) and syn- thesis of microbial protein (Breves and Schroder, 19911. Durand and Komisarczuk (1988) recommended that avail- able phosphorus (from dietary sources and salivary recy- cling) within the rumen should be at least 5 g/kg of organic matter digested to optimize degradation of cell walls from feeds by microbes. When cattle were fed 0.12 percent dietary phosphorus, ruminal fluid concentration was over 200 mg phosphorus/L, considerably greater than the 20 to 80 mg of phosphorus/L needed for maximum cellulose digestion in vitro (Hall et al., 1961; Chicco et al., 19651. This concentration typically is achieved in cattle by salivary recycling of phosphorus and from diets adequate to meet the animal's requirement. PHOSPHORUS UTILIZATION AND HOMEOSTASIS Net absorption of phosphorus occurs mainly in the small intestine (Grace et al., 1974; Reinhardt et al., 19881. Only small amounts are absorbed from the rumen, omasum, and abomasum. However, little is known about mechanisms and regulation of absorption anterior to the small intestine (Breves and Schroder, 19911. Absorption is thought to occur mainly in the duodenum and jejunum (Care et al., 1980; Scott et al., 19841. Unlike absorption of calcium, absorption of phosphorus is in direct relation to supply of potentially absorbable phosphorus in the lumen of the small intestine (Care et al., 19801. Presumably, as in nonru- minants, absorption occurs via two distinct mechanisms. A saturable vitamin D-dependent active transport system, separate and distinct from the active transport mechanism for Ca, is operative when animals are fed low phosphorus- containing diets. Synthesis of 1,25-dihydroxyvitamin D can be stimulated when blood phosphorus is very low resulting in more efficient absorption (Horst, 19861. Passive absorp- tion predominates when normal to large amounts of poten- tially absorbable phosphorus are consumed, and absorption is related directly to the amount in the lumen of the small intestine and to concentrations in blood plasma (Wasser- man and Taylor, 19761. Absorbed phosphorus may be retained or secreted (e.g., in milk) for productive functions or secreted into the lumen of the digestive tract for reabsorption or excretion in feces. Homeostasis of phosphorus is maintained predominantly by salivary recycling and endogenous fecal excretion, which are related directly to the amount of dietary phosphorus consumed and absorbed. Concentration of phosphorus in saliva can be 4 to 5 times of that in blood plasma. In cows, between 30 and 90 g of phosphorus is secreted daily into saliva (Reinhardt et al., 1988; Scott, 19881. Almost all phos- phorus in saliva is inorganic (Reinhardt et al., 1988), and the amount secreted appears to be regulated by parathyroid hormone (Wasserman, 19811. Inorganic salivary phospho- rus is absorbed across the intestine with equal or greater efficiency than dietary phosphorus (Challa et al., 19891. REQUIREMENT FOR ABSORBED PHOSPHORUS For the model, the requirement for absorbed phospho- rus was factorially derived by summing estimates of true requirements for maintenance, growth, pregnancy, and lactation. Maintenance Typically, 95 to 98 percent of total phospho- rus excretion is in feces. Three fractions are present that of dietary origin unavailable for absorption or not absorbed, that of endogenous origin which is inevitably excreted (inevitable fecal loss), and that of endogenous origin which is excreted to maintain homeostasis (representing phospho- rus absorbed by the intestine in excess of the need to maintain normal blood phosphorus). By definition, the maintenance requirement of phosphorus is the endoge- nous fecal loss (inevitable fecal loss) when phosphorus supply is just below or just meets the true requirement. In the past, the maintenance requirement was expressed

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Minerals 1 1 1 as a function of body weight (National Research Council, 1989a), based on fecal phosphorus excretion data extrapo- lated to zero phosphorus intake (Agricultural Research Council, 19801. This was later determined to be an inappro- priate approach (Agricultural and Food Research Council, 19911. Other workers suggested that inevitable fecal loss in ruminants was a function of total fecal dry matter (DM) excretion (Conrad et al., 1956; Preston and Pfander, 1964), which reflects the role of the salivary glands in phosphorus metabolism. It follows, therefore, that inevitable fecal loss of phosphorus also is related to DMI. The Agricultural and Food Research Council (1991) hypothesized that inevitable fecal loss of phosphorus is determined mainly by DMI, and not by live body weight. New research was available with cattle illustrating that a conceptually more sound and repeatable approach than expression as a function of body weight is to express maintenance requirement as a function of DMI when, by definition, dietary phosphorus is fed and absorbed very near the true requirement. Part of the maintenance requirement for absorbed phos- phorus of the animal is the inevitable fecal loss associated with microbial cells of the digestive tract which contain phosphorus and are excreted in feces. It is estimated that about half of the inevitable fecal loss of phosphorus is associated with microbial debris, and purines and pyrimi- dines of nucleic acids. This fraction can vary depending upon fermentability (fermented organic matter) ofthe diet. However, sufficient data are lacking to quantify this rela- tionship accurately (Kirchgessner, 19931. Klosch et al. (1997) fed growing bulls (228 or 435 kg BW) diets low (50 percent) or high (80 percent) in concen- trates and total phosphorus balance was determined. Net phosphorus retention was <1 g/animal per day, and fecal phosphorus excretion was not influenced by digestibility of organic matter consumed, or body weight. Total fecal phosphorus excretion (phosphorus of dietary origin not absorbed plus that of endogenous origin that was inevitably excreted) averaged 1.0 g/kg of DMI. The absorption coeff~- cient of total dietary phosphorus was assumed to be about 80 percent in the study of Klosch et al. (19971. Therefore, the absorbed phosphorus requirement for maintenance of growing animals was set at 0.8 g/kg of DMI in the current model. An additional 0.002 g/kg BW (Agricultural Research Council, 1980) of endogenous phosphorus loss from urine was considered part of the maintenance requirement for absorbed phosphorus in the model. Spiekers et al. (1993) fed a low phosphorus (0.21 per- cent) diet to two groups of lactating dairy cows of similar BW, but differing in daily milk yield (stage of lactation effect) and feed intake. For the two groups total phospho- rus intakes were 37 and 21.5 g/day, respectively; and, phos- phorus balance was similar and slightly negative, indicating that animals were fed below or very near the true require- ment. Total excretion of fecal phosphorus differed between groups (20.3 versus 13.3 g/cow per day) and was 51 percent greater per kg of body weight for cows at high versus low dietary phosphorus. However, calculated as a function of DMI, excretion of fecal phosphorus was 1.20 and 1.22 g/kg DMI per day for the high and low intake groups, respectively. It is estimated that the absorption coefficient of total dietary phosphorus for cows fed very close to the true requirement is 80 percent. Therefore, in the current model the maintenance requirement for nonlactating preg- nant and lactating cows was set at 1.0 g/kg of dietary dry matter consumed. A small amount of endogenous phospho- rus is inevitably excreted in urine. To account for this, an additional 0.002 g/kg BW (Agricultural Research Council, 1980) is considered as part of the maintenance requirement for absorbed phosphorus in the model. Growth The requirement for growth is the sum of the amount of absorbed phosphorus accreted in soft tissues plus that deposited in skeletal tissue. An accretion of 1.2 g of phosphorus/kg soft tissue gain was estimated by Agricul- tural Research Council (1980) and data of Grace (1983) from lambs confirmed this value. However, the majority of phosphorus deposition in growing animals is associated with new bone (hydroxyapatite) growth. Bone contains 120 g of calcium/kg and the theoretic accretion ratio of calcium-to-phosphorus is about 2.1 g calcium-to-1.0 g phosphorus (1.6 mol per 1.0 mol). Using this relationship and the accretion rate in soft tissues, the Agricultural and Food Research Council (1991) developed an allometric equation from data in the literature with growing cattle to describe the requirement for absorbed phosphorus for growth (g/kg average daily gain): P (g/day) = (1 2 + (4.635 x MW022) (BW-02211) x WG where MW = expected mature live body weight (kg), BW = current body weight, and WG = weight gain. Because bone is an early maturing component of the body, the allometric equation reflects declining require- ment for absorbed phosphorus for growing animals. This equation was used to define the absorbed phosphorus requirement for growing dairy cattle. For example in the model, for an animal with M = 681 kg, the absorbed phosphorus requirements (g/kg average daily gain) ranges from 8.3 g at 100 kg live BW (C) to 6.2 g at 500 kg. Pregnancy Quantitatively the requirement for phospho- rus for pregnancy is low until the last trimester. New infor- mation on accretion of phosphorus in conceptuses (fetus, fetal fluids and membranes, placentomes and uterine tis- sues) of 18 multiparous Holstein cows slaughtered at vary- ing times from 190 to 270 days of gestation was available (House and Bell, 19931. Changes in fetal mass and phos- phorus content across the sampling period were similar

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112 Nutrient Requirements of Dairy Cattle to earlier data (Ellenberger et al., 19501. Therefore, the requirement for absorbed phosphorus to meet demands of the conceptus for any day beyond 190 days of gestation is described in the model by the exponential equation: absorbed phosphorus (g/d) = 0. 02743e (o 05527 - o~oooo75 t)t 0.02743e (0 05527-0 000075 (t- l))(t- 7 where t = day of gestation (House and Bell, 19931. Estimates of rates of phosphorus accretion in concep- tuses of Holstein cows increase from 1.9 g/d at 190 to 5.4 g/d at 280 days of gestation, respectively. This equation should not be used to predict phosphorus accretion of the conceptus prior to 190 days of gestation. The phosphorus requirement of the conceptus at <190 days of gestation is very small and was set to zero in the model. Lactation The requirement for absorbed phosphorus (g per day) for lactation is equal to daily milk yield multi- plied by the percentage of phosphorus in milk. The phos- phorus content of milk ranged from 0.083 to 0.085 percent (Wu et al., 2000), 0.087 to 0.089 percent (Spiekers et al., 1993), and 0.090 to 0.100 percent (Flynn and Power,19851. The value of 0.090 percent (0.90 g of phosphorus per kg of milk) was used to compute requirements for absorbed phosphorus in the model. This is the same as that used by the working groups in Scotland and the United Kingdom (Agricultural and Food Research Council, 1991), France (Gueguen et al., 1989), and Germany (Kirchgener, 19931. In the last edition of this publication (National Research Council, 1989), the requirement of phosphorus for lacta- tion was adjusted depending upon fat content of milk. However, the phosphorus in cows' milk is distributed as: 20 percent esterif~ed to casein; 40 percent as colloidal inorganic calcium phosphate; 30 percent as phosphate ions in solution; and, only about 10 percent associated with the lipid fraction (;[enness and Patton, 1959; Renner, 19831. Therefore, an adjustment based on milk fat content is not of major quantitative and practical significance in defining the phosphorus requirement for lactation of dairy cows. DIETARY REQUIREMENT AND EFFICIENCY OF ABSORPTION The dietary requirement is the sum of the requirements for absorbed phosphorus for maintenance, growth, preg- nancy, and lactation divided by the absorption coeff~cientts) for phosphorus from the diet. The absorption coefficient in the denominator of the factorial equation potentially has more influence on the final computed dietary requirement than any of the single or combined requirement values for absorbed phosphorus. The smaller the absorption coeffi- cient, the greater will be the calculated dietary require- ment. In the last edition, an overall absorption coefficient of 50 percent was used (National Research Council, 1989b). Other working groups established overall values of 58 per- cent (Agricultural and Food Research Council, 1991), 60 percent (NRLO, 1982), 60 percent (Gueguen et al., 1989), and 70 percent (Kirchgessner, 19931. As with calcium, a single overall absorption coefficient was not considered appropriate for all types of feedstuffs, supplemental min- eral sources, or diets fed to various classes of dairy animals because of the known variation in absorption coefficients. The model evaluates the absorbable phosphorus content of the diet by determining the phosphorus available for absorption from each ingredient of the diet and comparing the sum of total phosphorus in the diet with the require- ment for absorbed phosphorus of the animal. To accurately determine the true absorption coeti~c~ent from a particular foodstuff or mineral source, phosphorus must be fed in an amount less than the animal's true requirement. This is to insure maximum efficiency of absorption of all potentially absorbable phosphorus from that particular source. A1SO7 especially with phosphorus, the amount of endogenous phosphorus recycled via saliva must be taken into account. This is most appropriately done experimentally by quantifying recycling with a tracer (e.g., p32) Most studies do not satisfy these experimental specifications. Thus, the true absorption coefficient is gen- erally unknown and the value given is an underestimation of true absorption. Apparent absorption of phosphorus (or apparent digestibility) determined in many studies is lower (largely because of copious endogenous fecal excretion) and not equivalent to the true absorption coefficient. If apparent absorption estimates are used to compute a dietary requirement, gross over-estimation results. Based on available data, absorption coefficients of phos- phorus used in the model for most foodstuffs commonly fed to cattle of various physiologic states were: 90 percent for calves consuming milk or milk replacer; 78 percent for young ruminating calves 100 to 200 kg body weight. True absorption coefficients for phosphorus from alfalfa hay or corn silage were 67 percent or 80 percent, respectively, for lactating cows yielding about 33.6 kg of 3.5 percent fat-corrected milk and consuming 21.7 kg DM daily (Martz et al., 19901. Using a tracer technique, Lotgreen and Kleiber (1953, 1954) reported the true absorption coeff~- cient of phosphorus in alfalfa hay fed to lambs ranged from 0.81 to 0.96. In the model, absorption coefficients of 64 percent and 70 percent were used for forages and concen- trates, respectively. More complete data are available to estimate absorption coefficients of various potential supplemental mineral sources (Table 15-41. These values were tabulated from Soares (1995b) and Peeler (1972), and other sources in the literature and used in the model. Those values deter- mined with ruminants, and especially with cattle, were given preference whenever possible in tabulation.

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Minerals 1 13 Dicalcium phosphate (calcium phosphate dibasic) with a true absorption coefficient of 75 percent in cattle (Tillman and Brethour, 1958; Challa and Braithwaite, 1988), phos- phoric acid with true absorption coefficient of 90 percent in cattle (Tillman and Brethour, 1958), and monosodium phosphate with a true absorption coefficient of 90 percent in sheep (Tillman and Brethour, 1958) were taken as refer- ence standards. The absorption coefficients of phosphorus in other mineral sources were set based on these reference standards and data where relative differences in phospho- rus absorption among these and other sources were esti- mated in various experiments (Soares, 1995b). Because sufficient studies with appropriate tracers are not available to estimate true absorption coefficients for most foodstuffs fed to lactating dairy cattle, an alternate approach would be useful. One such approach involves utilizing experimentally derived phosphorus balance data and the assumption that an accurate estimate ofthe mainte- nance requirement for absorbed phosphorus is 1.0 g/kg of DMI (Spiekers et al., 1993) plus endogenous urine output (0.002 g/kg BW; Agricultural Research Council, 19801. A calculated absorption coefficient can be derived as: Etrue requirement for maintenance (g per day) plus milk phos- phorus output (g per day) plus phosphorus balance (g per day)] divided by total phosphorus intake (g per day). The fecal output value from the actual balance determination is ignored because it represents unabsorbed dietary phos- phorus plus excess endogenous phosphorus which has been recycled to the digestive tract via saliva and excreted in feces. Using this approach, the calculated absorption coeffi- cients of phosphorus in mixed diets fed to lactating cows ranged from 67 to 100 percent (Morse et al., 1992b; Spiek- ers et al., 1993; Brintrup et al., 1993; Wu et al., 20001. In each study, two or three different concentrations of dietary phosphorus were fed. Within each study the calculated absorption coefficient declined as the dietary phosphorus concentration increased, as would be expected (Challa et al., 19891. A1SO, among three studies in which dietary phos- phorus concentrations (0.39 to 0.42 percent) most closely supplied the requirement of lactating cows, the calculated absorption coefficients L67 percent, Brintrup et al. (19931; 74 percent, Morse et al. (1992b); 72 percent, Wu et al. (20001] were similar to the overall absorption coefficient (70 percent) set by the German working group (Kirchges- sner, 19931. In the case of Spiekers et al. (1993), in which lactating cows were fed diets with 0.21 percent phosphorus (phosphorus-def~cient diet which resulted in slightly nega- tive phosphorus balance) the calculated absorption coeff~- cient was about 100 percent, as would be expected. This relationship is corroborated by regression of the calculated absorption coefficients on dietary phosphorus concentra- tions ranging from 15 to 62 percent, dry basis. Regression analysis (adjusted for number of experimental observations per treatment mean) was performed with a data set of 71 treatment means from 20 phosphorus balance trials (Hibbs and Conrad, 1983; Martz et al., 1990; Morse et al., 1992b; Spiekers et al., 1993; Brintrup et al., 1993; Wu et al., 2000; Rodriguez, 19981. The regression equation is: calculated absorption coefficient= 1.86696 - 5.01238(dietaryphos- phorus percent) + 5.12286 (dietary phosphorus percent)2; (r2 = 0 70) Based on the regression equation, the calcu- lated absorption coefficient was 1.0 with 0.22 percent phos- phorus and declined to a minimum absorption coefficient of 0.64 with 0.49 percent dietary phosphorus. All of these calculated absorption coefficients are greater than that (0.5) used by the National Research Council (19891. Efficiency of absorption of phosphorus depends upon a number of factors: age (or body weight) of the animal; physiologic state (e.g., nonlactating versus lactating); amount of DM or phosphorus intake; calcium-to-phospho- rus ratio; dietary concentrations of aluminum, calcium, iron, magnesium, manganese, potassium, and fat; intestinal pH; and, source of phosphorus (e.g., forages, concentrates, inorganic mineral supplements, and salivary phosphorus) (Irving, 1964; Peeler, 1972; Agricultural and Food Research Council, 1991; Soares, 1995b). EFFECT OF INTAKE OF PHOSPHORUS Efficiency of absorption of phosphorus declines as intake of phosphorus increases in cattle (Challa et al., 1989) and in sheep (Field et al., 1977~. However, over a considerable range of phosphorus intakes within recommended amounts the efficiency of absorption (absorption coefficient) from inorganic sources remained high and relatively constant in cattle (83 percent; Challa et al., 1989) and in sheep (74 percent; Braithwaite, 1986~. Because salivary phosphorus typically supplies appreciably more (e.g., at least two-fold greater amounts) phosphorus to the lumen of the small intestine than does dietary phosphorus, the efficiency of absorption of salivary phosphorus is important. Salivary phosphorus is in the form of inorganic phosphate salts with sodium and potassium. Over a considerable range of phosphorus intakes in tracer studies, the absorption coefficient of salivary endogenous phosphorus recycled to the small intestine was 68 percent to 81 percent in bull calves (Challa et al., 1989~. Excessive dietary phosphorus relative to the requirement reduced the efficiency of absorption of inorganic or salivary phosphorus (Braith- waite, 1983,1986; Challa et al., 1989~. EFFECT OF DIETARY CALCIUM Effect of increasing dietary calcium on phosphorus absorption was investigated where dietary calcium-to-phos- phorus ratios ranged from 0.6 to 3.6 (Field et al., 1983~. Efficiency of absorption of phosphorus in sheep was reduced by 18 percent with increasing amounts of calcium;

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114 Nutrient Requirements of Dairy Cattle amounts of calcium and phosphorus were within those amounts recommended by Agricultural Research Council (19801. At higher than recommended supplemental cal- cium, greater depression of phosphorus absorption would be expected (Agricultural and Food Research Council, 19911. Phosphorus deficiency was exacerbated in lambs fed diets supplying 1.5 times daily requirements for calcium (Seville and Ternouth, 1981), likely a result of reduced soluble phosphorus in the digestive tract (Wan-Zahari et al., 19901. PHYTATE PHOSPHORUS About two-thirds or more of phosphorus in cereal grains, oilseed meals, and grain by-products is bound organically in phytate; stems and leaves of plants contain very little phytate phosphorus (Nelson et al., 19761. Phytate phospho- rus is only slightly available or totally unavailable to non- ruminants (Soares, 1995b; National Research Council, 19981. However, inherent phytase activity of ruminal microorganisms renders nearly all of the phytate phospho- rus available for absorption (Reid et al., 1947; Nelson et al., 1976; Clark et al., 1986; Morse et al., 1992a; Ingalls and Okemo, 1994; Herbein et al., 19961. VARIATION IN PHOSPHORUS CONTENT OF FEEDS Phosphorus is the most expensive macromineral element supplemented in diets of dairy cattle. Therefore, laboratory analyses of feeds for phosphorus content is critically impor- tant for precise and accurate diet formulation to meet requirements at least cost. There is considerable variation in actual phosphorus content within types of forages and concentrates fed to dairy animals (Adams, 1975; Kertz, 19981. Estimates of variation (standard deviations) in phos- phorus content of many commonly used foodstuffs are given in Table 15-1 of this publication. GROWTH AND MILK YIELD RESPONSES TO VARYING DIETARY PHOSPHORUS CONCENTRATIONS In addition to the factorial approach for deriving the absorbed and dietary requirements, results of feeding trials in which varying dietary concentrations of phosphorus were fed to growing calves and lactating cows were evaluated. GROWING CALVES Huffman et al. (1933) concluded that 0.20 percent dietary phosphorus was not sufficient for growth of dairy heifers from 3 to 18 months of age. Maximum weight gains of dairy calves from 90 to 125 kg BW occurred when dietary phosphorus content was 0.24 percent, dry basis (Wise et al., 19581. However, bone ash content was greater when 1. . . .. dietary phosphorus was 0.33 percent compared with 0.24 percent, but greater phosphorus intake did not improve any other performance variables. Noller et al. (1977) found no differences in BW gain, efficiency of converting feed to gain, or concentrations of phosphorus in blood of Holstein heifers gaining between 0.68 to 0.82 kg/head per day when fed diets containing either 0.22 or 0.32 percent phosphorus. In a second trial, 0.32 percent compared with 0.22 percent dietary phosphorus increased concentrations of phospho- rus in serum, but no differences in weight gain or efficiency of feed conversion were observed. Increasing dietary phos- phorus from 0.24 to 0.31 percent (dry basis) increased DMI, average daily gain, breaking strength of ribs and tibia, and concentrations of inorganic phosphorus in blood plasma of dairy calves (Teh et al., 19821. Langer et al. (1985) compared 0.24, 0.3O, and 0.36 percent dietary phos- phorus fed to growing calves and found over the 10-week study that 0.30 percent resulted in maximum feed intake, average daily gain, and concentrations of phosphorus in blood plasma; no additional benefits were detected with 0.36 percent phosphorus. Miller et al. (1987) fed diets containing 0.08, 0.14, 0.2O, or 0.32 percent phosphorus and concluded, from concentrations of phosphorus in blood plasma and average daily gains, that at least 0.32 percent phosphorus was needed for heifers to gain 0.75 kg per day. Two sources (monoammonium phosphate and dicalcium phosphate) of phosphorus each used to give three dietary phosphorus concentrations (0.26, 0.34, and 0.41 percent, dry basis) were compared with growing dairy calves (;Jack- son et al., 19881. Increasing dietary phosphorus from 0.26 to 0.34 percent increased feed intake, body weight gain, concentrations of inorganic phosphorus in blood plasma, and bending moment of the tibia and rib. Body weight gain (0.94 kg/head per day) of calves fed 0.34 percent phosphorus was about 13 percent greater than that of calves fed 0.26 percent dietary phosphorus. Only plasma concen- tration of phosphorus was increased further with 0.41 per- cent phosphorus compared with lower concentrations. All responses were similar between sources of supplemental phosphorus. Based on all of these studies, 0.30 to 0.34 percent dietary phosphorus was sufficient for normal blood concentrations of phosphorus in blood, maximum average daily gains, and greater bone strength of growing dairy calves. LACTATING CATTLE Research literature was reviewed to find all possible results characterizing lactational responses to varying dietary concentrations of phosphorus. Phosphorus is often fed in greater dietary concentrations than needed to meet the requirement established in the current model. Is feed- ing phosphorus in excess of requirement beneficial?

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Minerals 1 15 Nine studies, with dietary phosphorus concentrations ranging from 0.24 to 0.65 percent of dietary dry matter, fed for periods ranging from the first 8 weeks of lactation to as long as three consecutive lactations, with average milk yields ranging from 15 to 40/kg per cow per day were examined to try to answer this question. Overall, supplying more dietary phosphorus than that calculated to meet the dietary requirement did not increase DMI or milk yield in any of the studies. The study of Kincaid et al. (1981) suggested that increasing dietaryphos- phorus increased DMI and 3.5 percent fat-corrected milk yield. However, based on the description of the analysis of variance in that paper the data were improperly ana- lyzed, thus invalidating interpretation. In one other study, feed intake and milk yield were lower for cows fed 0.24 versus 0.32 or 0.42 percent phosphorus (Call et al., 19871. Within none of the other studies was DMI or milk yield increased by increasing dietary phosphorus from its lowest concentration to a higher concentration (Stevens et al., 1971; Carstairs et al., 1981; Brodison et al., 1989; Brintrup et al., 1993; Dhiman et al., 1996; Wu and Satter, 2000; Wu et al., 20001. Milk fat and protein percentages were not attectect by concentration of dietary phosphorus in most studies. Milk protein percentage increased as phosphorus increased from 0.32 or 0.42 percent compared with 0.24 percent (Call et al., 19871. Protein content of milk was higher with 0.45 versus 0.35 percent phosphorus in the study of Wu and Satter (20001. Milk fat percentage was higher in year 1 of the study of Brodison et al. (1989) with 0.44 versus 0.35 percent phosphorus, but lower in the study of Brintrup et al. (1993) with 0.33 versus 0.39 percent phosphorus. There were no consistent effects of dietary phosphorus concentration on milk composition among studies. Concentrations of phosphorus in blood were evaluated in seven of the nine studies. The normal concentrations of inorganic phosphorus in plasma is 4.0 to 6.0 mg/dl for adult cattle (Goff, 1998a). In only one case among all of the studies was phosphorus in blood below the normal range (3.6 mg/dl for cows fed 0.24 percent dietary phospho- rus; Call et al., 19871; 0.24 percent did not provide the dietary requirement. In other studies, increasing dietary phosphorus increased the concentration of phosphorus in blood within or above the normal range. The DMI and milk yield of cows during early lactation were maximized with 0.40 to 0.42 percent dietary phospho- rus, and greater concentrations (0.50 to 0.52 percent) did not increase DMI or milk yield (Carstairs et al., 1981; Wu et al., 20001. Milk yield was not affected by the concentra- tion of the phosphorus in the diet during the first month, but from week 5 through 12 of lactation, it tended to be greater with 0.40 percent compared with 0.50 percent phosphorus (Carstairs et al., 19811. For the entire 84-d treatment period, cows fed 0.40 percent phosphorus pro- duced 8 percent more milk than those fed 0.50 percent phosphorus. Feeding 0.42 percent phosphorus to high yielding cows during the first 8 weeks of lactation maxim- ized milk production, and resulted in positive phosphorus balance and normal concentrations of phosphorus concen- trations in blood serum (Wu et al., 20001. Based on results of nine studies, a concentration in the range of 0.32 to 0.42 percent phosphorus for the entire lactation was sufficient, depending upon milk production potential of the cows and nutrition supplied. No benefits on lactational performance of dietary concentrations >0.42 percent phosphorus were reported in any short- or long- term studies which were properly analyzed. Daily dietary requirement determined by the factorial method is expressed as g per cow per day, and not as a percentage of the diet. Therefore, supplying the require- ment requires a reasonably accurate estimate of actual DMI. FREE-CHOICE PHOSPHORUS Coppock et al. (1972, 1975) studied the practice of free- choice feeding of phosphorus-containing supplements to dairy heifers and lactating cows to meet requirements when diets were low or marginally deficient in phosphorus or calcium. With heifers there was little relationship between need for the mineral elements and free-choice consump- tion of dicalcium phosphate or defluorinated phosphate. For lactating cows offered basal diets providing phosphorus and calcium below requirements for 9 and 12 weeks, there was no evidence that cows consumed dicalcium phosphate to correct the deficiency or that appetite for phosphorus and calcium supplements coincided with the animals' nutri- tional requirements. PHOSPHORUS DEFICIENCY Detailed description of occurrence, etiology, clinical pathology, diagnosis, treatment, and prevention of phos- phorus deficiency in ruminants has been described by Goff (1998a). Signs of deficiency may occur rather quickly if dietary phosphorus is insufficient. Deficiency is most com- mon in cattle grazing forages on soils low in phosphorus or in animals consuming excessively mature forages or crop residues with low phosphorus content (less than 0.25 per- cent, dry basis). Nonspecific chronic signs of deficiency include unthriftiness, inappetence, poor growth and lacta- tional performance, and unsatisfactory fertility; but signs are often complicated by coincidental deficiencies of other nutrients such as protein or energy. Animals maybe chroni- cally hypophosphatemic (low phosphorus in blood plasma 2 to 3.5 mg/dl), but the concentration of phospho- rus in milk remains within the normal range. In severe deficiency cases, bone mineral mass is lost, and bones

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Mineral influences upon urea utilization and cellulose digestion by rumen microorganisms using the artif~cial rumen technique. J. Anim. Sci. 10:693-697. Burton, J., B. Mallard, and D. Mowat. 1993. Effects of supplemental chromium on immune responses of periparturient and early-lactation dairy cows. J. Anim. Sci. 71:1532-1539. Call, J. W., J. E. Butcher, J. L. Shape, R. C. Lamb, R. L. Woman, and A. E. Olson. 1987. Clinical effects of low dietary phosphorus concentrations in feed given to lactating cows. Am. J. Vet. Res. 48:133-136. Call, J. W., J. E. Butcher, J. T. Blake, R. A. Smart, and J. L. Shape. 1978. Phosphorus influence on growth and reproduction of beef cattle. J. Anim. Sci. 47:216-225. Calvert, C. C., and L. W. Smith. 1972. Arsenic in milk and blood of cows fed organic arsenic compounds. J. Dairy. Sci. 55:706-714. Cantley, L. C. Jr., L. Josephson, R. Warner, M. Yanagisawa, C. Lechene, and G. Guidotti. 1977. 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